1. Field of the Invention
This invention relates to chemical and physical cyclic processing systems which undergo cyclic temperature change. This temperature change usually significantly limits the processing capability of the system. Thermocyclic processing systems cover a very broad spectrum, ranging from physical pressure-swing adsorption/desorption of carbon dioxide by activated carbon at temperatures of 100.degree. F. for cleaning natural gas to chemical reduction/oxidation of metal oxides at temperatures of 1500.degree. F. for oxygen enrichment of air and low NO.sub.x cyclic combustion of natural gas at 2000.degree. F. According to the present invention, the thermo-cyclic process is conducted in thermal exchange relation with a sufficient quantity of thermal absorption/release material to operate the process substantially isothermally at about the transition, such as phase-change, temperature of the thermal absorption/release material during exothermic and endothermic portions of the thermocyclic process.
2. Description of the Prior Art
The adsorption of gas or vapor is always associated with the evolution of heat in an active adsorption zone. This evolution of heat causes a temperature rise which significantly limits the sorptive capacity of the system. In normal operation, a high-temperature wave progresses through a sorbent bed in advance of the gas or vapor adsorptive wave so that the bed temperature and thus the exit gas temperature rises before the adsorption break-point or sorptive capacity is reached. Cooling coils have been placed in such adsorbers to remove the heat of adsorption which reduces the temperature rise and makes the sorbent action more nearly isothermal. Use of cooling coils in adsorbers has produced considerable increase in sorptive capacity. However, due to poor thermal exchange in sorption beds, an extensive and costly heat exchange system is required. Furthermore, heat removed during the adsorption segment of the cycle must be replaced during desorption or regeneration portion of the cycle. This is usually accomplished by application of external heat during regeneration, such as using hotter regeneration gases. Thus, the overall process is less thermally efficient. Because of the additional capital costs for heat exchange surface and additional operating costs for supply of external regeneration heat, the use of cooling coils is very limited in commercial practice.
A more common practice is to use a much larger sorbent bed to reduce the temperature rise due to adsorption as described in Kohl, A. L. and Riesenfeld, F. C., Gas Purification. 4th Ed. Gulf Publishing Company, Houston, TX, Chapter 12, Gas Dehydration and Purification by Adsorption, 645-648, 1985. In this method, the additional sorbent acts as a heat sink, as well as a sorbent, reducing the peak temperature of the temperature wave. This, coupled with the extra sorbent in the bed, compensates somewhat for the loss of sorbent capacity due to temperature rise. If large excesses of sorbent are used, much of the heat of sorption can be stored within the bed in this manner. However, since heat is stored using the sensible heat capacity of the sorbent, a temperature rise sufficient to store this heat must still occur, and this temperature rise reduces sorptive capacity of the sorbent. Molecular sieve adsorbent beds use this principle for drying organic-water azeotropes, such as ethanol/water under conditions to store much of the heat of adsorption during the drying portion of the cycle and using the stored heat during the regeneration portion of the cycle as described in Garg, D. R. and Yon, C. M., "Adsorptive Heat Recovery Drying System," Chem. Eng. Progr., 54-60, February 1986.
Yang and Cen, in "Improved Pressure Swing Adsorption Processes for Gas Separation by Heat Exchange Between Adsorbers and by High-Heat-Capacity Inert Additives," Ind. Eng. Chem. Proc. Des. Dev. 25, 54-59, 1986, have suggested by model simulation that an inert solid material with a high sensible heat capacity be added to the adsorbent bed to reduce the need for excess sorbent. Yang and Cen, in their model simulation, added various amounts of iron particles to a bed of activated carbon that was used to separate 50/50 hydrogen/methane (H.sub.2 /CH.sub.4) and 50/50 hydrogen/carbon monoxide (H.sub.2 /CO) mixtures by adsorption. The use of 20 weight percent of iron resulted in reduction of the peak temperature of the activated carbon bed from about 100.degree. C. to about 50.degree. C., thereby improving the separation of 50/50 H.sub.2 /CO mixtures. This use of a high sensible heat capacity material reduces the magnitude of the temperature changes that occur during the adsorption/desorption cycle, but cannot reduce the temperature changes as far as desired since the storage of heat as sensible requires temperature change to effect storage.
Several other sorption-heat related methods exist. U.S. Pat. No. 4,341,539 teaches a thermally regenerative desiccant element of micron size silica gel held within an expanded web of fluoro plastic elastomer which is bonded onto a heat conductive plate which is cooled by a stream of cooled air to remove the heat of sorption. A number of patents disclose sorbent improvement with various additives: U.S. Pat. No. 2,255,041 teaching calcium chloride as a hygroscopic agent incorporated with calcium hydrosilicate and/or calcium hydroaluminate skeleton chemically anchoring the hygroscopic agent to the skeletal network to result in a structure of considerable resilience; both U.S. Pat. Nos. 4,366,090 and 2,292,632 teach a support material of an alkali metal silicate into which is incorporated an adsorbent material to result in good mechanical strength; U.S. Pat. No. 2,625,516 teaching improving drying properties by coating calcium sulfate with alkali metal silicate to prevent disintegration; and U.S. Pat. No. 2,986,525 teaching fusing of adsorbent salts, such as alkali and alkaline earth metal salts, to form a eutectoid for improved physical properties of a refrigerant adsorbent.
The use of silica gels as heat storage materials is known, and the addition of an inorganic heat-of-fusion salt to fumed silicon dioxide is taught by International Patent Application Publication No. WO 80/01073. Thermal storage using solid/liquid phase change chemicals is taught using a large number of inorganic salts by U.S. Pat. No. 4,421,661 and 4,512,388.
However, none of the prior art known to the applicant suggests use of thermal absorption/release materials which undergo transition, such as phase change, to greatly reduce and nearly eliminate temperature change in otherwise thermo-cyclic systems.